First-principles spectroscopy of multiparticle excitations in low-dimensional materials*

The synthesis of low-dimensional materials, such as monolayer transition metal dichalcogenides (TMDCs), opened the door to the study of new classes of systems with spatial confinement, locked spin and valley physics, weak electronic screening, and enhanced many-electron interactions. Such systems host a variety of charged and neutral multiparticle excitations – such as plasmons, excitons, trions, biexcitons, and condensates – often displaying strong and well-defined spectroscopic signatures. We present here a first-principles formalism and calculations based on the interacting Green’s function to compute and understand these excitations and their dynamics.

We first discuss plasmons in metallic monolayer TMDCs and predict that they are long-lived and display a unique and universal dispersion not found in the ideal 2D electron gas due to the broken continuous translational symmetry. Such plasmons can also support a giant enhancement of the electric field intensity of the order of 10⁷ and serve as platforms for practical applications involving light-matter interactions, such as photocatalysis. For semiconducting monolayer TMDCs, our parameter-free formalism and calculations elucidate the nature of strongly bound biexcitons in such systems. Such biexcitons, with a computed binding energy of ~ 20 meV, display a nontrivial wavefunction in reciprocal space. Finally, we also discuss how exciton-exciton interaction can be used to understand ultrafast exciton dynamics in few-layer TMDCs, which were recently reported in time-resolved spectroscopy experiments.